The Trump administration on Monday announced that it would change the way the Endangered Species Act is applied, significantly weakening the nation’s bedrock conservation law credited with rescuing the bald eagle, the grizzly bear and the American alligator from extinction.

The changes will make it harder to consider the effects of climate change on wildlife when deciding whether a given species warrants protection. They would most likely shrink critical habitats and, for the first time, would allow economic assessments to be conducted when making determinations.

The rules also make it easier to remove a species from the endangered species list and weaken protections for threatened species, a designation that means they are at risk of becoming endangered.

Overall, the new rules would very likely clear the way for new mining, oil and gas drilling, and development in areas where protected species live.

Interior Secretary David Bernhardt said the new rules would modernize the Endangered Species Act and increase transparency in its application. “The act’s effectiveness rests on clear, consistent and efficient implementation,” he said in a statement Monday.

Commerce Secretary Wilbur Ross said in a statement the revisions “fit squarely within the president’s mandate of easing the regulatory burden on the American public, without sacrificing our species’ protection and recovery goals.”

The new rules are expected to appear in the Federal Register this week and will go into effect 30 days after that.

Environmental groups denounced the changes as a disaster for imperiled wildlife. A recent United Nations assessment, they noted, has warned that human pressures are poised to drive one million species into extinction and that protecting land and biodiversity is critical to keep greenhouse gas emissions in check.

Climate change, a lack of environmental stewardship and mass industrialization have all contributed to the enormous expected global nature loss, the United Nations report said.

Ever since President Richard M. Nixon signed the Endangered Species Act into law 1973, it has been the main United States legislation for protecting fish, plants and wildlife, and acted as a safety net for species on the brink of extinction. The peregrine falcon, the humpback whale, the Tennessee purple coneflower and the Florida manatee all would very likely have disappeared without it, scientists say.

Republicans have long sought to narrow the scope of the law, saying that it burdens landowners, hampers industry and hinders economic growth. Mr. Bernhardt wrote in an op-ed last summer that the act places an “unnecessary regulatory burden” on companies.

They also make the case that the law is not reasonable because species are rarely removed from the list. Since the law was passed, more than 1,650 have been listed as threatened or endangered, while just 47 have been delisted because their populations rebounded.

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Over the past two years Republicans made a major legislative push to overhaul the law. Despite holding a majority in both houses of Congress, though, the proposals were never taken up in the Senate. With Democrats now in control of the House, there is little chance of those bills passing.

The Trump administration’s revisions to the regulations that guide the implementation of the law, however, mean opponents of the Endangered Species Act are still poised to claim their biggest victory in decades.

One of the most controversial changes removes longstanding language that prohibits the consideration of economic factors when deciding whether a species should be protected.

Under the current law, such determinations must be made solely based on science, “without reference to possible economic or other impacts of determination.”

Gary Frazer, the assistant director for endangered species with the United States Fish and Wildlife Service, said that phrase had been removed for reasons of “transparency.” He said the change leaves open the possibility of conducting economic analyses for informational purposes, but that decisions about listing species would still be based exclusively on science.

Environmental groups saw a danger in that. “There can be economic costs to protecting endangered species,” said Drew Caputo, vice president of litigation for lands, wildlife and oceans at Earthjustice, an environmental law organization. But, he said, “If we make decisions based on short-term economic costs, we’re going to have a whole lot more extinct species.”

The new rules also give the government significant discretion in deciding what is meant by the term “foreseeable future.” That’s a semantic change with far-reaching implications, because it enables regulators to disregard the effects of extreme heat, drought, rising sea levels and other consequences of climate change that may occur several decades from now.

When questioned about that change and its implications in the era of climate change, Mr. Frazer said the agency wanted to avoid making “speculative” decisions far into the future.

Among the animals at risk from this change, Mr. Caputo listed a few: Polar bears and seals that are losing crucial sea ice; whooping cranes whose migration patterns are shifting because of temperature changes; and beluga whales that will have to dive deeper and longer to find food in a warmer Arctic.

Jonathan Wood, a lawyer at the Pacific Legal Foundation, a conservative group that has represented landowners in opposing endangered species designations, said he believed the changes would improve the law by simplifying the regulatory process and making the law less punitive.

“It’s a shift away from conflict in favor of more collaboration and cooperation,” he said.